Fluorine is chemically the most active of all the elements. Therefore, fluorine as well as its compounds (e.g., nitrogen trifluoride) is widely used in various fields.
In the nuclear power industry, fluorine is used as a raw material for producing uranium hexafluoride (UF6) (which is employed for concentration of uranium) and also as a raw material for producing sulfur hexafluoride (SF6) (which is employed as a high dielectric constant gas). Further, in the semiconductor industry, fluorine is used as a gas for a dry washing or etching of the surface of silicon wafers by taking advantage of the properties of fluorine such that it reacts with silicon oxide coating and selectively reacts with impurity metals contained in silicon. In addition, in other industries, fluorine is used to control the gas permeability of a high density polyethylene which is employed as a material for a gasoline tank, and used to improve the wettability of olefin polymers. Olefin polymers are processed using a gaseous mixture of fluorine and oxygen, thereby introducing a carbonyl fluoride group (—COF) into the surface of the olefin polymers. A carbonyl fluoride group can be easily converted into a carboxyl group by a hydrolysis reaction (which is caused by, e.g., the moisture in the air), thereby improving the wettability of the olefin polymers.
On the other hand, nitrogen trifluoride (NF3) has received much attention, since the time it was used in large amounts as a fuel/oxidant for rockets for planetary explorations which were planned and executed by the National Aeronautics and Space Administration (NASA) of the U.S.A. At the present day, in the semiconductor industry, nitrogen trifluoride is used in large amounts as a dry etching gas in the semiconductor manufacturing processes, and as a CVD chamber cleaning gas in the semiconductor manufacturing processes and liquid crystal display manufacturing processes. As a CVD chamber cleaning gas, a perfluorinated compound (PFC), such as carbon tetrafluoride (CF4) or ethane hexafluoride (C2F6), is also used, but it has recently been found that a PFC is greatly promoting the global warming phenomenon. For this reason, the use of a PFC is likely to be restricted or banned at the global level by, e.g., the Kyoto Protocol. Thus, more and more nitrogen trifluoride is being used as a substitute gas for a PFC.
As described hereinabove, fluorine and nitrogen trifluoride are widely used in various fields. Therefore, it is important to efficiently produce fluorine or nitrogen trifluoride on a commercial scale.
Fluorine is produced exclusively by an electrolytic method, since it reacts with many substances so easily that it cannot be isolated by the conventional chemical oxidation method or the conventional substitution method. In the electrolytic method, fluorine is produced usually by using as an electrolysis liquid a hydrogen fluoride-containing molten salt of potassium fluoride (KF) and hydrogen fluoride (HF) wherein the molar ratio of KF to HF is 1/2 (which is hereinafter frequently referred to as an “HF-containing molten salt of a KF-2HF system”).
On the other hand, the methods for producing nitrogen trifluoride are classified into a chemical method and an electrolytic method. In the chemical method, fluorine is first obtained by electrolysis using as an electrolysis liquid an HF-containing molten salt of a KF-2HF system, and then the fluorine is reacted with, e.g., a metal fluoride ammonium complex, thereby obtaining nitrogen trifluoride. In the electrolytic method, nitrogen trifluoride is produced directly by using as an electrolysis liquid an HF-containing molten salt of ammonium fluoride (NH4F) and hydrogen fluoride (HF), or an HF-containing molten salt of ammonium fluoride, potassium fluoride (KF) and hydrogen fluoride.
In general, from the viewpoint of ease in machining process and a reduction in the conductor resistance, it is desired that a metal is used as a material for the electrodes of an electrolytic apparatus. However, in an electrolytic apparatus for producing fluorine or nitrogen trifluoride by using a hydrogen fluoride-containing molten salt, it is unsuitable to use a metal as an anode. The reason for this is that if a metal anode is used in the electrolysis of a hydrogen fluoride-containing molten salt for producing fluorine or nitrogen trifluoride, the metal will be dissolved vigorously, thus generating a metal fluoride sludge or forming a passivation layer which stops the current, thus rendering it impossible to continue the hydrolysis.
For example, in the electrolytic production of fluorine, if nickel is used as an anode, the nickel is corroded and dissolved vigorously during the electrolysis, thus forming a large amount of nickel fluoride sludge. Likewise, in the electrolytic production of nitrogen trifluoride, if nickel is used as an anode, the nickel will be corroded and dissolved vigorously during the electrolysis, thus forming a large amount of nickel fluoride sludge.
Thus, when the electrolytic production of fluorine or nitrogen trifluoride is conducted by using a metal as an anode and using a hydrogen fluoride-containing molten salt as an electrolysis liquid, the metal will be dissolved vigorously, thus forming a metal fluoride sludge. For this reason, it is necessary to regularly change electrodes and electrolysis liquids, thus rendering it difficult to continuously produce fluorine or nitrogen trifluoride. Further, if the current density is increased, dissolution of the metal is markedly increased, rendering it difficult to conduct the electrolysis at a high current density.
Therefore, in the electrolytic production of fluorine or nitrogen trifluoride by using a hydrogen fluoride-containing molten salt as an electrolysis liquid, carbon is conventionally used as an anode. However, the use of carbon as an anode causes the following problems.
First, the case of fluorine production is described. When fluorine is produced using carbon as an anode and using a hydrogen fluoride-containing molten salt (such as an HF-containing molten salt of a KF-2HF system) as an electrolysis liquid, the fluorine generation reaction represented by formula (1) below is caused by an electric discharge of a fluoride ion on the surface of the anode while generating graphite fluoride ((CF)n)) by the reaction represented by formula (2) below. The surface energy of graphite fluoride is extremely low due to the presence of covalent C—F bonds therein, so that the wettability of graphite fluoride with the electrolysis liquid is poor. Graphite fluoride is decomposed by Joule heat into carbon tetrafluoride (CF4), ethane hexafluoride (C2F6) or the like, as shown in the reaction represented by formula (3) below.
If the reaction rate of the reaction of formula (2) below (i.e., the graphite fluoride generation reaction) is higher than that of the reaction of formula (3) below (i.e., the graphite fluoride decomposition reaction), the surface of the carbon electrode will be coated with graphite fluoride, thus causing a decrease in the wettability of the carbon electrode with the electrolysis liquid, resulting in the stop of the current (the anode effect). A high current density increases the reaction rate of the reaction of formula (2) below, thereby promoting the anode effect.HF2−→(1/2)F2+HF+e−  (1)nC+nHF2−→(CF)n+nHF+e−  (2)(CF)n→xC+yCF4, zC2F6, etc  (3)
As described below, a high concentration of water in the electrolysis liquid also promotes the anode effect. As shown in formula (4) below, the carbon at the surface of the carbon electrode reacts with water in the electrolysis liquid to generate graphite oxide (CxO(OH)y). Graphite oxide is so unstable that it undergoes a substitution reaction with atomic fluorine which is generated by an electric discharge of a fluoride ion, wherein the substitution reaction converts the graphite oxide into graphite fluoride ((CF)n), as shown in formula (5) below (the atomic fluorine is generated as an intermediate product and, finally, converted into graphite fluoride). Further, the interlayers of the graphite are broadened by the generation of graphite oxide, thus promoting the diffusion of fluorine in the interlayers, resulting in an increase in the reaction rate of the reaction of formula (2) above (the graphite fluoride generation reaction). Thus, the anode effect is promoted.xC+(y+1)H2O→CxO(OH)y+(y+2)H++(y+2)e−  (4)CxO(OH)y+(x+3y+2)F−→(x/n)(CF)n+(y+1)OF2+yHF+(x+3y+2)e−  (5)
The occurrence of the anode effect decreases the wettability of the anode with the electrolysis liquid, thus reducing the production efficiency drastically. Hence, the occurrence of the anode effect poses a great problem in the use of carbon as an anode. For preventing the anode effect, it is required not only to perform a complicated operation, such as reducing the water concentration of the electrolysis liquid by dehydration electrolysis, but also to adjust the electrolytic current density to a level lower than the critical current density at which the anode effect occurs. The critical current density of a widely used carbon electrode is about 10 A/dm2. A 1 to 5 weight % incorporation of a fluoride (such as lithium fluoride or aluminum fluoride) into the electrolysis liquid increases the critical current density. However, even by this method, the critical current density still remains at most about 20 A/dm2.
On the other hand, in the case of nitrogen trifluoride production by electrolyzing a hydrogen fluoride-containing molten salt, using carbon as an anode, there also arise the same problems as described above. As mentioned above, the methods for producing nitrogen trifluoride can be classified into a chemical method and an electrolytic method.
In the chemical method, as described above, fluorine is first obtained by electrolysis, and then the fluorine is reacted with, e.g., a metal fluoride ammonium complex, thereby obtaining nitrogen trifluoride. When this method is employed, the problem of the occurrence of the anode effect is encountered in the step of producing fluorine by electrolysis.
In the case of electrolytic production of nitrogen trifluoride by using carbon as an anode, an HF-containing molten salt of ammonium fluoride (NH4F) and hydrogen fluoride (HF), or an HF-containing molten salt of ammonium fluoride, potassium fluoride (KF) and hydrogen fluoride, is used as an electrolysis liquid. When this method is employed, the anode effect is encountered, as in the case of fluoride production using carbon as an anode and using an HF-containing molten salt of a KF-2HF system as an electrolysis liquid.
In addition, a problem arises in that carbon tetrafluoride (CF4) and ethane hexafluoride (C2F6), which are generated by the reaction of formula (3) above (the graphite fluoride decomposition reaction), decrease the purity of nitrogen trifluoride which is the desired product. Nitrogen trifluoride, carbon tetrafluoride and ethane hexafluoride are extremely similar to each other with respect to the physical properties, thus rendering it difficult to separate them from each other by distillation. Therefore, it is necessary to employ a costly purification method for obtaining high purity nitrogen trifluoride.
Thus, the conventional method for producing fluorine or nitrogen trifluoride by electrolyzing a hydrogen fluoride-containing molten salt, using carbon as an anode, poses the problem of the occurrence of the anode effect. As described above, for preventing the anode effect, it is required not only to perform a complicated operation, such as reducing the water concentration of the electrolysis liquid by dehydration electrolysis, but also to adjust the electrolytic current density to a level lower than the critical current density at which the anode effect occurs.
Therefore, it has been desired to develop an electrolytic apparatus which can be operated without the occurrence of the anode effect even at a high current density and without the occurrence of an anodic dissolution.    [Patent Document 1] Unexamined Japanese Patent Application Laid-Open Specification No. Hei 7-299467    [Patent Document 2] Unexamined Japanese Patent Application Laid-Open Specification No. 2000-226682    [Patent Document 3] Unexamined Japanese Patent Application Laid-Open Specification No. Hei 11-269685    [Patent Document 4] Unexamined Japanese Patent Application Laid-Open Specification No. 2001-192874    [Patent Document 5] Unexamined Japanese Patent Application Laid-Open Specification No. 2004-195346    [Patent Document 6] Unexamined Japanese Patent Application Laid-Open Specification No. 2000-204492    [Patent Document 7] Unexamined Japanese Patent Application Laid-Open Specification No. 2004-52105    [Patent Document 8] Japanese Patent No. 364545    [Patent Document 9] Unexamined Japanese Patent Application Laid-Open Specification No. 2005-97667    [Non-Patent Document 1] “Fusso Kagaku To Kogyo (I): Shinpo To Oyo (Fluorine Chemistry and Industry (I): Progress and Application)”, edited by Nobuatsu WATANABE, published in 1973 by The Kagaku Kogyo Ltd., Japan    [Non-Patent Document 2] “Fusso Kagaku To Kogyo (II): Shinpo To Oyo (Fluorine Chemistry and Industry (II): Progress and Application)”, edited by Nobuatsu WATANABE, published in 1973 by The Kagaku Kogyo Ltd., Japan    [Non-Patent Document 3] “Diamond Electrochemistry”, edited by Akira FUJISHIMA, published in 2005 by BKC Inc., Japan